Direct drive servo systems are finding wide application in robotics type systems. A servo system using a direct drive motor simplifies the mechanical system by eliminating backlash, increasing reliability and decreasing maintenance problems caused by gears, belts and couplings. Many conventional servo motor systems, however, include a number of operating problems as well as unnecessary manufacturing costs and have undesirable weight and space requirements.
The torque of a conventional reluctance type motor, for example, is proportional to the square of the flux density. This makes the torque output of the motor non-linear with respect to the input signal, thereby increasing the complexity of the circuitry required to process the error signal. Still another problem is that other servo motors require bipolar currents and this increases both the complexity of the driving amplifiers and the power supplies. Some require, for example, a full bridge rectification system which suffers from reliability problems through cross conduction at high power levels because of variable transistor delays and reversed biasing problems. Often, such power supplies require a large power transformer which adds to the size, weight and cost of the servo system. Still other common problems suffered by prior art servo systems are the effect of ripple current in the motor and AC line isolation.
Prior art position sensors coupled to the motor, such as a synchro/resolver, require both primary and secondary windings and have mechanical features, such as slip rings, which increase maintenance and reliability problems. Also, prior art synchro/resolvers used in servo systems do not have as high a resolution or as accurate as is often desired in digital direct drive servo positioning systems. In some cases, the output signal of the synchro/resolver encounters interference with the motor windings.
Still another problem of prior art servo systems is that the commutation of the motor is not proportional. This is to say that the amount of power which must be supplied to the motor as the load increases is not uniform for all positions of the motor shaft. Thus, the requirements of the motor input signal by way of waveform shape, duration and phase vary depending on the shaft position of the rotor of the motor. Prior art resolvers do not provide any compensation for this problem.